Not for the first time, the prime minister was happily promoting the irreconcilable. "By stepping up, showing leadership and competing with the world," he announced last week, "the UK can prove that there need not be a tension between green and growth".

It could have been worse. After the Treasury and the business department tried to scupper the UK's long-term carbon targets, David Cameron stepped in to rescue them. The government has now promised to cut greenhouse gases by 50% by 2027, which means that, with a following wind, the UK could meet its legally binding target of 80% by 2050. For this we should be grateful. But the coalition has resolved the tension between green and growth in a less than convincing fashion: by dumping responsibility for the environmental impacts on someone else.

The carbon cut we have made so far, and the carbon cut we are likely to make by 2027, have been achieved by means of a simple device: allowing other countries, principally China, to run polluting industries on our behalf.

Officially, the UK's greenhouse gas emissions have fallen from 788 million tonnes in 1990 to 566 million tonnes in 2009. Unofficially, another 253 million tonnes should be added to our account. That's the difference between the greenhouse gases released in manufacturing the goods we export and those released in manufacturing the goods we import. The reason our figures look better than those of most other nations is that so much of our manufacturing industry has moved overseas. It is this which allows the government to meet its targets. If the stuff we buy is made in China, China gets the blame.

This would be less of an issue if China were obliged to restrict its emissions. But under the only global treaty in force at the moment – the Kyoto protocol – developing countries have no need to reduce their impacts. That suits the governments of both rich and poorer nations. Governments like ours can pretend there is no conflict between green and growth. They avoid unpopular decisions, allowing people to consume whatever they fancy, and they keep business sweet by promising endless expansion. Governments like China's can keep supplying us with the goods we couldn't produce at home without breaking our obligations.

Though the UK has been better served by this system of false accounting than most nations, it's a global problem. A recent paper in the Proceedings of the National Academy of Sciences shows that even if all the unenforceable pledges made in the Copenhagen accord were honoured, carbon cuts by the rich nations between now and 2020 would be cancelled out by their imports from developing countries. In other words, nothing will have been achieved. Already, 16bn tonnes – equivalent to 27 years of the UK's emissions – have gone missing from the international system as a result of this false accounting.

One of the results of such prestidigitation is that we greatly underestimate the pollution caused by manufacturing. Because we don't see these emissions in our own accounts, and because the government seeks to clean up only the carbon which officially belongs to us, we have concentrated on greening the electricity and heating in homes and offices, and the transport we use, and have overlooked everything else we buy. We emphasise services and forget about goods.

As soon as you account for the stuff we use that's made elsewhere, you discover that we've been missing more than half the story. As the Guardian's carbon calculator shows, manufacturing and consumption is responsible for 57% of our real emissions. Cameron's contention that there need not be a tension between green and growth collapses.

It looks naive today, and it will look cynical in 2027. The new carbon target is likely to be more than annulled by the growth in outsourced greenhouse gases. Figures produced by the Carbon Trust, a government-funded body, show that by 2025 our true emissions (produced both here and abroad) will rise from 844 million tonnes to 908 million tonnes. So much for the 50% cut by 2027.

This is an issue which governments of all varieties appear to have ignored: no official body has yet produced a public report on the subject. But recent freedom of information requests by Guy Shrubsole and Alex Randall of the Public Interest Research Centre show that civil servants have long been advising ministers that the issue makes a mockery of their carbon targets. Their briefings have warned that "the rise in UK consumption has outstripped the improvements achieved" and that "the government needs to be cautious about over-claiming on its achievements in decoupling economic growth from environmental degradation". Some hope.

The obvious conclusion is that the international system for regulating emissions is bust. It has been bust since it was launched in 1992. From the outset it allowed rich and poor nations tacitly to conspire in producing a semblance of action. As soon as the UN tried to resolve this problem by bringing developing nations into the system at Copenhagen in 2009, the process collapsed.

After the talks fell apart there, we were promised that the matter would be settled at Cancún the following year. It wasn't, so we were told to wait until the talks in Durban in December 2011. At the end of last month, both the EU and US negotiators announced that it isn't going to happen in Durban either, but that miracles will be administered in 2012. I can picture the delegates at the 2100 meeting, sitting beside the air-conditioning units in Murmansk, promising that it will all be sorted out in 2101 at Ny-Ålesund.

If governments are serious about preventing climate breakdown, they must admit the process has failed, abandon it and start again with a new one. I still believe the most promising system is the cunningly-designed carbon trading scheme proposed by Oliver Tickell in his book Kyoto2. This would also be resisted, as it doesn't allow rich countries to dump their emissions on poorer ones. But it has two advantages in expediting the negotiations, over the current system. The first is that it is fair, transparent and universal, with fewer opportunities for false accounting: governments are less likely to feel cheated. The second is that, because greenhouse gases would not be allocated to states, there is no longer an incentive (which has fatally undermined the negotiations to date) to pass the buck to other countries and engage in carbon nationalism.

The old system is dead, but there will be no successor until governments are brave enough to issue the certificate. Until that point, there's only one thing these interminable, soul-sapping talks will save: face.